In various devices which include microprocessor controlled circuits, different timing clock frequencies are needed for different applications. In such devices, the correct clock must be connected to the microprocessor with jumpers before initial power-up of the circuit. If at a later time the clock frequency needs to be changed, the circuit must be powered down and the jumpers must be changed to connect the microprocessor to a clock having the desired new frequency. Such a procedure is awkward and time consuming, and requires that operation of the circuit be terminated during the change-over. It would be clearly advantageous if the change in frequencies could be accomplished automatically and without the need to terminate operation of the circuit.